The present invention relates to an electrophotographic printer such as a laser printer and an apparatus for electrophotography such as an electrophotographic copying machine. Specifically, the present invention relates to optimum transfer control of a transferring means for transferring toner images to paper.
Conventionally, a transferring means for transferring toner images in the above described kind of apparatus for electrophotography transfers toner images at a toner image carrier to paper by passing paper as a recording medium through an interval between a transferring roller, whereon a transferring voltage is charged, and the toner image carrier.
Transferring means of the above described type has a problem that transferring performance varies depending on changes in resistant values of the recording medium and transferring rollers in accordance with change in an environmental condition. FIG. 11 is a graph indicating a transferring characteristic relating to humidity which is one of the environmental conditions. A transferring voltage with which an optimum transferring efficiency can be obtained under an environmental condition at room temperature and normal humidity (20.degree. C., 60%) is set, and the transferring efficiency at the transferring voltage is designated as 100%. And, transferring efficiencies which are determined at various humidities such as a low humidity on clear day and a high humidity on rainy day are indicated. The characteristic graph reveals that transferring efficiency decreases remarkably at humidities shifted from the environmental humidity for which the transferring voltage is set.
In consideration of the above problem, conventional transferring means of the above described type prevents the decreasing of the transferring efficiency by detecting resistant values of the recording medium and by controlling the transferring voltage charged to a transferring roller as disclosed in JP-A-55-28081 (1980), JP-A-2-300774 (1990), and JP-A-4-190381 (1992). Further, JP-A-64-40867 (1989) discloses a method wherein a discharging voltage is controlled in accordance with a transferring current which is determined under a condition wherein both the transferring device and the discharger are concurrently operated.
However, currently, a demand for recording to various kinds (such as materials and sizes) of recording media by electrophotographic printers or electrophotographic copiers is increasing, and such a problem has been created that the control in the conventional transferring means can not satisfy the demand sufficiently.
Especially, a transferring defect caused in a case using paper for an over-head projector (OHP) at high temperature and high humidity is one of the problems.
The paper for OHP is processed for anti-static treatment by decreasing surface resistance to prevent the paper from becoming incapable of being feed by mutual electrostatic adhering when the paper is fed from a paper cassette. The above processing causes another problem such as extremely decreasing the transferring efficiency by flowing current from a charging device such as a transferring roller through the paper for OHP to the AC discharger at a high humidity condition and lowering the voltage at transferring points (refer to FIG 12).
Further, a method which uses needle electrodes for discharging electricity on paper without using high AC has the problem that discharging to the needle electrodes causes the lowering of the voltage at the transferring points and thus the transferring efficiency can not be improved.
Furthermore, another method, wherein a discharging voltage is controlled in accordance with a measured transferring current when both a transferring voltage and the discharging voltage are concurrently supplied, has problems that a measured transferring current is influenced by the discharging voltage and results of the measurement can not be obtained exactly, transfer can not be performed stably, and paper-wrapping is caused as a result of insufficient discharging of electricity.